The present invention relates generally to methods and apparatus for molding concrete and other moldable compositions and, more particularly, to methods and molds for forming concrete into arched supports for bridges and the like.
Bridges and overpasses are essential components of any roadway system. The building of bridges is one of the oldest engineering tasks still in practice, and an almost infinite variety of techniques and materials have been employed. The oldest bridge still in use is an oval type that is more than 2,200 years old.
Today, the most common bridge is a column and truss structure, using pre-cast concrete beams as the load bearing mechanism. The useful lifespan of today""s bridges is relatively short in spite of the costs. Fifty years, by any standard, is a short useful lifespan of a bridge structure, and many have not achieved even that durability due to the deterioration of the bridge""s infrastructure.
The basic purpose of a bridge is to form and hold a roadbed stationary while spanning a natural or man-made water channel or road. Planning the actual shape and dimensions of the bridge takes into account the intended use, existing physical features, and maximum xe2x80x9cextremexe2x80x9d anticipated flow in the drainage channel beneath.
These factors can be calculated manually. However, there are numerous computer programs designed to model the anticipated flows and desired shapes in a channel that take into account the variables in a drainage area. For a discussion of variables in flow through a culvert or bridge and guidelines for cross-section areas for channels, see Normann, J. M., 1985 (Hydraulic Design Series 5, NTIS publication PB86196961).
The local variables of water flow are usually known or available. The bridge cavity over the channel must carry all the water flow coming under the structure, up to maximum extreme conditions, unless backwaters above the bridge are acceptable. In some applications, it may be practical to have excess water flow over the roadbed in extreme conditions. One design provides a large arch for normal and anticipated flow and smaller arches on each side for unusual and extreme conditions.
Concrete is commonly used to form beams and piers and other components of today""s bridges and overpasses. It is used as a covering material for the steel framework and roadbed. In the xe2x80x9ccolumn and trussxe2x80x9d type bridge structure, the road surface is designed in a tension configuration. Deterioration of the steel reinforcing material, due to chemical reactions within the load bearing members and improper bonding, is the principal cause for bridges requiring repairs or replacement.
A very large number of bridges in the United States interstate system have failed to be useful for the expected lifespan of the structure. Poor construction practices were followed in some cases. However, the major problem lies in the basic tension design, where the load is vertical or down and the support is acting at 90 degrees or horizontally in tension.
Even with the advancements in bridge building techniques presently available, there remains a need for a technique that will produce an adequate structure at a lower cost. There is a need for a technique that will allow such structures to be produced using simple manual labor and without requiring large cranes or other expensive and dangerous equipment. There continues to be a need to produce structures using concrete, because of its low cost and availability, but which will be long-lasting. Still further, there is a need for a system that will permit structures that can be custom built to accommodate local terrain and that will be aesthetically pleasing.
The present invention is directed to a mold assembly. The assembly comprises a plurality of elongate elements stacked parallel to each other in multiple rows and in gravity-stable arrangement to form an upper contour of a selected shape.
Still further, the present invention includes a method for molding a moldable composition. The method comprises stacking a plurality of elongate elements parallel to each other in multiple rows and in gravity-stable arrangement to form an upper contour of a selected shape. Unset moldable composition is spread over the upper contour of the stacked elongate elements. The moldable composition is allowed to set up, and the stacked elongate elements are removed.